The present invention relates to electronic circuits having one or more contactors and, particularly, to supplemental circuits for dissipating or preventing arcing across such contactors.
In certain DC circuits, it is necessary to include a device to interrupt the flow of current through the circuit. One device for accomplishing current interruption is called a “contactor.” The contactor includes a set of electrical contacts where at least one of which is movable. For example, electrical circuits connecting a power supply (e.g., a battery) to an electric motor typically include a solenoid-driven contactor or “solenoid” to start and stop the motor by permitting and interrupting current flow through the circuit and, thus, to the motor.
In general, contactors permit current flow between the contacts when contact surfaces thereof are abutted together (i.e., the contactor is “closed”). Additionally, current flow is terminated or prevented when the contacts are moved away from each other to spaced apart locations (i.e., the contactor is “open”). In other words, separation of the previously closed contacts breaks the continuous conduction path and creates an opening in the circuit. However, the current does not immediately cease flowing across the contactor. Rather, an “arc” or discharge of current occurs across the gap between the opening contact surfaces. The arc or discharge occurs due to the following two conditions. First, stored energy in an inductive DC circuit prohibits an instantaneous change in current and causes a sharp voltage rise between the contacts upon opening. The sharp voltage rise forces the current to “jump the gap” between the contacts. Second, as the last portion of the surface of the contactor is breaking, localized current concentration causes the contact material to melt creating a metallic plasma that carries the arc current as the contact surfaces continue to separate.
Typically, both of the above-discussed factors occur substantially simultaneously during a typical contact opening. The net result is an arc characterized by a sharply rising voltage waveform between the contacts. During the opening period of about one millisecond, the voltage rises to tens or hundreds of volts.
Arcing is undesirable because it causes erosion and pitting of the contacts of the contactor. Eventually erosion and pitting cause failures. Either the contacts fail to abut together or touch (referred to as a “no make” condition) or they become stuck together (i.e., become “welded contacts”). Of the two possible failure modes, the welded contacts condition is often more serious since, thereafter, the contactor is incapable of interrupting the circuit.